1. Field of the Invention
This invention relates in general to the operation of a magnetic storage device, and more particularly to method, apparatus and program storage device for sensing increased resistance changes in an MR element to detect MR sensor events.
2. Description of Related Art
The “Magnetoresistive (MR) effect was discovered in perfect-crystal samples exposed to very high magnetic fields. The effect was also recently discovered in sputtered metallic thin films consisting of magnetic layers a few nanometers thick separated by equally thin nonmagnetic layers (Giant Magnetoresistive elements, or “GMR”). A large decrease in the resistance of these films is observed when a magnetic field is applied. The cause of this effect is the spin dependence of electron scattering and the spin polarization of conduction electrons in ferromagnetic metals. With layers of the proper thickness, adjacent magnetic layers couple antiferromagnetically to each other with the magnetic moments of each magnetic layer aligned antiparallel to the adjacent magnetic layers.
Magnetoresistive (MR) heads are employed in magnetic disc drives to read data from the storage disc. More particularly, the MR head employs an MR element whose resistance changes with changes in the confronting magnetic field. As the disc rotates adjacent the read head, changing magnetic fields due to recorded data on the disc moving past the MR element induces changes in the resistance of the MR element. A fixed bias current is applied to the head to generate a voltage across the head representative of the data. The voltage changes with the resistance changes to provide signals representing the data.
There is a continuing need to increase the recording capacity, and hence the density of data recording, on discs. Consistent with this need, efforts have been directed to more narrow data tracks and smaller track spacing. As track widths become narrower and track spacing becomes smaller, the read heads also become narrower, so as not to extend over the space between the tracks that might cause errors due to simultaneous reading of two or more data tracks.
The MR sensor is also sensitive to temperature changes that occur due to friction during head/disk interaction. Thermal asperities caused by the MR head contacting the surface of the recording media causes the instantaneous temperature to rise. This rise in temperature causes the data signal to spike and may momentarily disrupt the recovery of data from the drive if not compensated for. The resistance of the MR sensor is proportional to this temperature change, which may be caused by MR sensor events.
The flying heights of MR heads continue to decrease in an effort to provide ever increasing areal densities. For example, current flying height targets are less than 7 nm. Thus, it is critical to determine if unintentional contact between the head and disk occurs to eliminate thermal asperities and, more importantly, to maintain long-term mechanical reliability of the head/disk interface. However, currently an accurate way for detecting contact between the MR head and the recording media is not available without incurring additional cost and complexity to provide some type of contact sensor.
In addition, seek loss occurs in disk drives when servo is lost and the voice coil motor (VCM) seeks uncontrollably between the inner-diameter (ID) and the outer-diameter (OD) crash stop-this uncontrollable seek may cause significant disk damage. A quick recovery of this erratic VCM motion is possible if a sensor can detect when the head stack is ON the load/unload ramp, therefore allowing a control algorithm to reset the VCM's position there before continuing normal operation.
It can be seen then that there is a need for a method, apparatus and program storage device for sensing increased resistance changes in MR element to detect MR sensor events.